HalogenBonding: Unifying Perspectives on Organicand Inorganic Cases.

Wefind for distinct classes of halogen bonded complexes (MF3X···Y) that the ab initio BSSE-correctedbinding energies (ΔE) andenthalpies (ΔH) are predictedby functions of the form y = A/rn+ C. Here X is a halogen atom, Y is a base, ris the X···Y separation, and A, n, and Care constants. The actualvalue of n(5.5 < n< 7.0for ΔE) for each class isdetermined evidently by the availability of the lone pairs on thebase and is insensitive to M such that all of the complexes of a givenbase fall on the same curve for yversus r. Remarkably, several bases show the same behavior in somecases such that just three curves account for 55 MF3I···Ycomplexes of 11 bases, where M = C, Si, Ge, Sn, and Pb. Two additionalbases, THF and NF3, which form especially strong and weakcomplexes, respectively, are in classes by themselves. Anomalous modesof halogen bonding are identified; in particular, furan forms sigma-holecomplexes via carbons 2 and 3 (through the π system) in thering in preference to the oxygen site. These results are in line withexperimental observations for furan–dihalogen complexes, andseveral other small MF3I···Y pairs are proposedin this work for experimental interrogation. Instead of halogen bonding,CF4tends to form weak sigma-hole bonds to bases via thepolarized central carbon atom, and new examples of such pro-dativeinteractions to carbon in CF4are identified in this work.We find that GeF3I and SnF3I form I···Yhalogen bonds of comparable energies to those formed by the smallerand better studied CF3I. PbF3I forms the strongesthalogen bond regardless of the identity of the base; SiF3I consistently forms the weakest link. [ABSTRACT FROM AUTHOR]